The present invention relates to ink jet printers. It finds particular application in conjunction with a thermal ink jet printhead, and will be described with particular reference thereto. It is to be appreciated, however, that the invention finds further application in conjunction with other ink jet technologies, such as hot melt or phase change piezo ink jet, as well as microfluid transport devices used in biological, chemical, and pharmaceutical applications.
Thermal ink jet printing is generally a drop-on-demand type of ink jet printing, which uses thermal energy to produce a vapor bubble in an ink-filled channel that expels a droplet. A thermal energy generator, typically a resistor, is located in each of the channels at a predetermined distance from the nozzles. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble. As the bubble grows, the ink bulges from the nozzle, but it is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble begins to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle. This results in the separation of the bulging ink as an ink droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides momentum and velocity to the droplet in a substantially straight-line direction towards a recording medium, such as paper.
High-performance, high-speed thermal ink jet printheads generate large quantities of heat, especially during extended high-density printing, such as when the printhead completely covers a page with ink. The ink droplet ejecting performance of thermal ink jet printheads is temperature dependent, and as such, print quality is adversely affected as the device heats up. Much of the heat created in thermal ink jet printheads during operation is waste heat that, if not properly dealt with, leads to print quality failure modes. In fact, at least two failure modes can be encountered as the result of undissipated waste heat. One of these failure modes is analogous to vapor lock in automobile engines. More particularly, in a thermal ink jet printhead stable bubbles of air and ink block the flow of ink into the ink channels and cause print defects related to lack of ink flow to the drop ejectors. A second failure mode occurs when the heater substrate, drop ejectors and ink adjacent thereto achieve too high of a steady state temperature. This results in premature boiling, which prevents the well-timed explosive boiling that ejects stable and appropriately sized ink droplets. As a result of the self-heating of the printhead, the volume of ink ejected in each droplet becomes greater due to the higher energy content of the ink, as well as the lower viscosity of the ink. The increased spot size resulting from the larger ink droplets lead to non-uniformity in a variety of print characteristics, such as optical density, color hue and saturation, and text character width.
Various devices and methods for reducing overheating of the heater substrate and overall printhead have been employed. Many of the prior art devices incorporate a heat sink of sufficient thermal mass and low enough thermal resistance that the device temperature does not rise excessively. For example, FIG. 1 shows a prior art printhead 10 where a first, lower silicon heater substrate 12 is bonded to a second, upper silicon channel substrate 14. The channel substrate 14 includes parallel grooves 11 formed in the bottom surface, which extend in one direction. When the channel substrate 14 is bonded to the heater substrate 12, channels 20 and nozzles 33 are formed at front face 22. The thermal ink jet die module (composed of heater substrate 12 bonded to channel substrate 14) is bonded directly to a heat sink substrate 13, and adjacent to a daughter board (not shown).
Typically, these heat sinks, such as the one shown in FIG. 1, are massive and problematic for long, high-area coverage print jobs. Often times, special measures are required to remove heat from the heat sink, which gradually accumulates heat and, accordingly, rises in temperature. These special measures, which include water and/or air cooling of the heat sink, add expense and take up accessible design space.
The present invention contemplates a new and improved ink jet printhead, which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a device for selectively applying droplets of at least one fluid to an associated medium includes a nozzle plate, which defines a plurality of fluid-emitting nozzles, and a heater substrate disposed adjacent and substantially perpendicular to the nozzle plate. The heater substrate has a rear surface, a front surface, a top surface, and a bottom surface, where the rear and front surfaces are substantially larger than the top and bottom surfaces. A fluid housing is attached to the nozzle plate. The fluid housing includes a fluid inlet for connecting to an associated fluid tank and a first internal wall, which defines a fluid flow path such that fluid flows from the fluid inlet substantially around all of the rear, top, and front surfaces of the heater substrate. An intermediate layer is disposed adjacent a portion of the front surface of heater substrate. The intermediate layer defines a plurality of fluid flow channels in fluid communication with the plurality of nozzles. A channel cap plate, which is disposed adjacent the intermediate layer, caps the plurality of fluid flow channels.
In accordance with another aspect of the present invention, a printhead for use with an ink jet printer includes a nozzle plate, which defines a plurality of ink-emitting nozzles, is disposed substantially parallel to an associated print medium. A heater substrate, which is disposed adjacent and substantially perpendicular to the nozzle plate, includes a plurality of heating elements. A printhead housing, which is attached to the nozzle plate, substantially surrounds the heater substrate. The printhead housing includes a first internal wall, which defines an ink flow path around the heater substrate. An ink flow channel defining layer, which is disposed adjacent a portion of the heater substrate, defines a plurality of ink flow channels in fluid communication with the plurality of nozzles.